This application claims the benefit of Japanese Patent Application P2002-32307, filed on Feb. 8, 2002, the entirety of which is incorporated by reference.
1. Field of the Invention
The present invention relates to a semiconductor light-emitting device, particularly to a semiconductor light-emitting device using a III nitride film, which can be preferably used for a photo diode or the like.
2. Related Art Statement
The III nitride film is used as a semiconductor film that constitutes the semiconductor light-emitting device, and has been expected in recent years as a high-brightness light source for light covering from green light to blue light and a light source for ultra-violet light and white light as well.
In recent years, a so-called epitaxial substrate that includes an underlayer film formed on a substrate by epitaxial growth is often used as a substrate on which such a III nitride film is formed. Then, a III nitride layer group, in which a single layer m nitride film or a plurality of III nitride films is laminated, is formed on the epitaxial substrate by using an MOCVD method or the like. A desired semiconductor light-emitting device is thus obtained.
FIG. 1 is a structural view showing an example of a conventional so-called PIN type semiconductor light-emitting device.
In a semiconductor light-emitting device 10 shown in FIG. 1, a buffer layer 2 made of GaN, an underlayer. 3 made of Si doped n-GaN, an n-type conductive layer 4 made of Si doped n-AlGaN, a light-emitting layer 5 having a multi-quantum well (MQW) structure made of InGaN, a p-type cladding layer 6 made of Mg doped p-AlGaN, and a p-type conductive layer 7 made of Mg doped p-GaN are formed in this order on a substrate 1 mainly composed of sapphire single crystal. In the semiconductor light-emitting device 10 shown in FIG. 1, layers from the n-type conductive layer 4 to the p-type conductive layer 7 constitute a light-emitting structure.
A part of the n-type conductive layer 4 is exposed, and an n-type electrode 8 made of Al/Ti or the like is formed in the exposed portion, and a p-type electrode 9 made of Au/Ni or the like is formed on the p-type conductive layer 7.
When a predetermined voltage is applied between the n-type electrode 8 and the p-type electrode 9, recombination of carriers occurs in the light-emitting layer 5 to emit light having a predetermined wavelength. Note that the wavelength is determined by the structure, the composition or the like of the light-emitting layer.
In the semiconductor light-emitting device 10 shown in FIG. 1, the buffer layer 2 serves as a buffering layer in order to complement the difference of lattice constants between the substrate 1 and the underlayer 3 to enable the underlayer 3 or the like formed above the substrate 1 to perform epitaxial growth. Therefore, the buffer layer is usually formed in an amorphous state under low temperature from 500xc2x0 C. to 700xc2x0 C. leaving the crystallinity out of consideration.
As a result, the buffer layer 2 contains a relatively large quantity of dislocations, and a part of the dislocations propagates as threading dislocations into the underlayer 3, n-type conductive layer 4, light-emitting layer 6, p-type cladding layer 6 and p-type conductive layer 7. This has led these layers to contain a dislocation quantity exceeding 1010/cm2, and crystal quality has deteriorated. Particularly, the above-described tendency is conspicuous in the case of the semiconductor light-emitting device for a short wavelength, that is, the case where the n-type conductive layer 3 and the light-emitting layer 4 contain large quantities of Al.
There has existed a problem that a so-called nonradiative recombination occurred due to the dislocation in the light-emitting layer and luminous efficiency reduced when layers of low crystal quality constituted the semiconductor light-emitting device under such high dislocation density.
An object of the present invention is to provide a semiconductor light-emitting device that reduces dislocation density and has a high luminous efficiency.
The present invention provides a semiconductor light-emitting device having a substrate, an underlayer made of nitride semiconductor and a light-emitting device structure including a nitride semiconductor layer group on the underlayer. The nitride semiconductor constituting the underlayer contains at least Al and has a dislocation density of 1011/cm2 or less. The nitride semiconductor layer group constituting the light-emitting device structure has an Al content smaller than that of the nitride semiconductor constituting the underlayer and a dislocation density of 1xc3x971010/m2 or less.
The inventors have intensely studied for achieving the above object. The above-described deterioration of luminous efficiency occurs as a result of the high dislocation due to the low crystal quality of each layer that constitutes the semiconductor light-emitting device. The inventors have attempted to improve the crystal quality by reducing the dislocation quantity of each layer that constitutes the semiconductor light-emitting device.
As described above, the dislocation in each layer that constitutes the semiconductor light-emitting device is caused by the buffer layer of low crystal quality. Sapphire single crystal has been used for the substrate 1 and n-GaN has been used for the underlayer 3 in the conventional semiconductor light-emitting device shown in FIG. 1. Such combination has induced a problem that dislocations in the underlayer 3 propagated into the n-type conductive layer 4 when the n-type conductive layer 4 made of n-AlGaN, which is a III nitride, was formed on the underlayer 3. Particularly, cracks often occurred when Al content in n-AlGaN, which constitutes the n-type conductive layer 4, increased. When the n-type conductive layer 4 contains Al, its lattice constant is reduced, and the cracks are caused by tensile stress induced in the n-type conductive layer 4.
Consequently, the inventors have had various kinds of studies not only on the buffer layer but also on the underlayer. In the conventional semiconductor light-emitting device shown in FIG. 1, the light-emitting layer is made of nitride semiconductor having Ga as main component as it is obvious from the device construction. Therefore, it has been taken for granted that the underlayer provided for these layers was made of material having Ga series, GaN for example, as main component. This resulted in lattice mismatch with the substrate, and the buffer layer formed under low temperature has been required to mitigate the mismatch.
However, the inventors have paid attention to the composition of the underlayer, which was taken for granted, and have attempted to change the component. As a result, the inventors have thought out to constitute the underlayer of nitride semiconductor having Al as the main component of low dislocation and good crystal quality. The underlayer complements a large difference of lattice constants and can perform epitaxial growth on the substrate such as sapphire even when the buffer layer does not exist. Further, the crystal quality of the conductive layer and the light-emitting layer formed on the underlayer is also improved and the dislocation quantity of the layers are reduced due to the high crystal quality of the underlayer. As a result, the dislocation quantity in each layer, which constitutes the semiconductor light-emitting device, is reduced and nonradiative recombination based on the high dislocation quantity can be effectively restricted.
Further, the underlayer of high crystal quality of this kind has superior heat dissipation characteristic. For this reason, the synergic effect of the high crystal quality and the heat dissipation characteristic of the underlayer can improve the luminous efficiency of the semiconductor light-emitting device, and high-brightness light emission can be performed.
These and other objects, features and advantages of the invention will be appreciated upon reading the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes of the same could be made by the skilled person in the art.